In general, hollow fiber membranes are prepared by using a non-solvent phase separation method or a thermally induced phase separation method. Hollow fiber membranes produced by the aforementioned method are classified into a microfiltration membrane (MF), a ultrafiltration membrane (UF), a nanofiltration membrane (NF), and a reverse osmosis membrane (RO).
A typical hollow fiber membrane has the outermost layer and the innermost layer, and these parts are responsible for the removal performance of a contamination source as an active layer. Further, the hollow fiber membrane exhibits a difference in permeability depending on the porosity of the hollow fiber membrane, and when the cross-sectional structure of the hollow fiber membrane has a low porosity, the hollow membrane has a low permeability and a high strength, but in contrast, when the structure has a high porosity, the hollow fiber membrane exhibits a high permeability, but shows a low strength.
Conventional hollow fiber membranes are intended to minimize the filtration pressure loss by increasing the porosity of the cross-section of the hollow fiber membrane in order to increase the permeability, but only result in a reduction in tensile strength and breaking strength of the hollow fiber membrane. In addition, efforts have been made to even remove fine contaminants by decreasing the porosity of the cross-section of the hollow fiber membrane in order to enhance the removal performance of the contaminants, but the filtration pressure loss is increased, so that the permeability rapidly decreases, and the economic loss was enormous in order to utilize this. Various methods have been tried to overcome this, but it has been difficult to increase the water permeability by minimizing the filtration pressure loss, and simultaneously improve the removal rate and the strength.